<div id="map"></div>

<script>
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        container: 'map',
        style: 'mapbox://styles/mapbox/light-v10',
        center: [-77.035, 38.875],
        zoom: 12
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    var geojson = {
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        'features': [
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                'geometry': {
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        // and the source must have the 'lineMetrics' option set to true
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            lineMetrics: true,
            data: geojson
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            source: 'line',
            id: 'line',
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                // 'line-gradient' must be specified using an expression
                // with the special 'line-progress' property
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            layout: {
                'line-cap': 'round',
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